1. Field of the Invention
The present invention relates to the vaporization of solids. More particularly, the present invention relates to an apparatus and method for vaporizing solid precursors used in chemical vapor deposition and/or atomic layer deposition processes.
2. Background of the Technology
Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are known techniques for forming solid films on a substrate by reaction of vapor phase chemicals near the surface of a substrate. In general, CVD and ALD techniques involve the delivery of gaseous reactants to the surface of a substrate where a chemical reaction takes place under temperature and pressure conditions favorable to the thermodynamics of the reaction. The type and composition of the layers that can be formed using CVD and/or ALD are limited by the ability to deliver the reactant(s) (otherwise known as precursor(s)) to the surface of the substrate. Various liquid precursors have been successfully used in CVD and/or ALD applications by delivering the liquid precursors in a carrier gas. Analogous attempts to deliver solid precursors to a CVD and/or an ALD reaction chamber have shown much less success.
In prior known solid precursor delivery devices, a carrier gas is passed through a heated container containing volatile solid precursor(s) at conditions conducive to vaporization of the solid. The carrier gas mixes with the vaporized solid and the vaporized solid is drawn from the container in a vacuum environment and carried with the carrier gas to the reaction chamber. Prior known solid precursor delivery procedures have been unsuccessful in reliably delivering solid precursor to the reaction chamber. For example, as the solid precursor is vaporized, the heat of vaporization needed to release the vaporized precursor molecules tends to cool underlying solid precursor molecules thus forming crystals, which tend to prevent or limit further vaporization of any underlying solid precursor.
Lack of control of solid precursor vaporization is, at least in part, due to the changing surface area of the bulk solid precursor as it is vaporized. Such a changing surface area when the bulk solid precursor is exposed to high temperature produces a continuously changing rate of vaporization, particularly for thermally sensitive compounds. This ever-changing rate of vaporization results in an inability to consistently contact the carrier gas with the solid material, which in turn results in a continuously changing and non-reproducible flow of vaporized solid precursor delivered for deposition in the reaction chamber. A predictable amount of precursor cannot therefore be delivered. As a result, film growth rate and composition of such films on substrates in the reaction chamber cannot be controlled adequately and effectively.
U.S. Pat. No. 5,447,569 discloses the use of a tube containing a plurality of longitudinal slits, wherein vaporization of solid material packed within the tube is controlled by moving the tube through a band of heaters, wherein the vaporized material exits the tube perpendicularly to the longitudinal axis of the tube via the slits.
U.S. Pat. No. 5,674,574 discloses the use of a rotatable surface contained within a container. The rotatable surface, which has solid precursor material applied thereon, is heated using a focused thermal beam as it rotates. The vaporized solid exits the container through an outlet and is delivered to a reaction chamber.
U.S. Pat. No. 6,072,939 discloses the use of a hollow tube-like container having a longitudinal axis passing through a first end. The hollow container is attached to an injector that is in communication with an inlet of a reaction chamber. The injector moves the hollow container through a heater that vaporizes the solid material contained therein.
As discussed above, prior art methods, which use of elaborate systems to vaporize the solid precursor, have numerous disadvantages. Acordingly, there is a need in the art for a simplified vapor delivery system for delivering solid precursors at a highly controllable rate without decomposition of the solid precursors during vaporization. There is a further need in the art to both easily and efficiently vaporize a solid precursor at a controlled rate such that a reproducible flow of vaporized solid precursor can be delivered to the reaction chamber.
The present invention is directed to an apparatus and method for vaporizing solid precursors that overcomes the problems of the prior art so as to provide a simple, more efficient apparatus and method for vaporizing solid precursors in the formation of thin layers on substrates.
The present invention provides an apparatus for vaporizing solid precursors. The apparatus includes a housing defining a sealed interior volume having an inlet for receiving a carrier gas, at least one surface contained in the housing having a solid precursor applied thereon, and a heating member for heating the solid precursor. Although the heating member may or may not be contained within the surface supporting or containing the solid precursor, the heating member is preferably contained in the surface or surfaces contained within the housing.
The present invention also provides for a method for vaporizing solid precursors. The method involves applying a solid precursor to a surface located within a housing having a sealed interior volume. The surface is then heated either directly or indirectly by a heating element until a sufficient temperature is reached to vaporize the solid precursor.
According to one embodiment of the present invention, the surface supporting or containing the solid precursor includes one or more heated baffles or rods. A heating member may be contained within the baffles or rods. Additionally, the baffles or rods may conform to the shape of the heating member. For example, the baffles or rods may be cone-shaped to fit tightly over conventional cone-shaped heaters.
According to another embodiment of the present invention, the surface for the solid precursor includes one or more heated meshes or gratings. A heating member may be contained within the mesh or grating. The mesh or grating may conform in shape to maximize flow through of carrier gas (for example, the mesh or grating may be s-shaped).
In a preferred embodiment of the present invention, the vaporized precursor is mixed with a carrier gas and delivered to a reaction chamber where the vaporized precursor is deposited on the surface of a substrate by conventional deposition methods.
With the foregoing and other objects, advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the preferred embodiments of the invention and to the appended claims.